The basic shell construction assemblies of a home - foundation, walls, floors and roofs - are covered in detail in this section. Standard house building practices are illustrated with the emphasis on high insulation levels and a continuous air/vapour barrier installation. Details include how the floor, wall and ceiling assemblies join (and the sealing problems created) and how airtightness and insulation levels are maintained in spite of obstructions such as windows, doors, wiring, plumbing, pipes, or chimneys. The object is not to cover all aspects of structural building design - only how energy efficient construction can be incorporated into existing residential construction practices.For reference, Table 2 lists metric building material sizes along with Imperial equivalents.
Controlling Heat Loss
Most important to the success of an energy efficient home is the quality of construction. Even poorly sited homes (as often occurs in urban areas), with little passive solar gain potential, can be very energy efficient homes. Adequate levels of insulation and careful sealing can combine to cut heat losses so that the energy required for space heating will only be 15% to 25% of a 'normally' constructed home.
A good way to think about a house is to consider it a 'shell' which must keep heat inside during the winter. This shell is made up of floors, ceilings and walls constructed with various building materials. Heat is lost from the inside of your home in two ways: either directly through the shell or when warm indoor air leaks out through cracks and holes (replaced by cold outside air leaking in).
Energy loss through the building shell can be 40% to 70% of the total and is controlled with insulation. Air leakage losses account for the remainder and is controlled by the air/vapour barrier, weatherstripping and caulking.
Insulation is measured by its R-value (or RSI-value). The higher the R-value, the better the insulation stops heat flow. R-values for different building materials are given in Table 1. The total R-value for a wall, ceiling or floor is the sum of the values of each part or layer.
For an Energy Efficient House in a cold climate (5000 heating degree days or less), the recommended R-values (RSI-values) are:
- R-10 (RSI 1.7) under foundation floor.
- R-30 (RSI 5.0) for above grade floors such as overhangs, cantilevers and below projecting windows.
- R-20 (RSI 3.5) for all walls above and below grade.
- R-40 (RSI 7.0) for all ceilings whether sloped or flat.
For a Super Energy Efficient House in a cold climate or if building in a very cold climate (5700 heating degree days or more), the recommended R-values (RSI-values) are:
- R-30 (RSI 5.3) for all foundation walls.
- R--36 (RSI 6.3) for all walls above grade.
- R-40 (RSI 7.0) for above grade floors such as overhangs, cantilevers and below projecting windows.
- R-60 (RSI 10.5) for all ceilings whether sloped or flat.
Most insulation products can be placed in one of three types - blanket, loose fill or rigid.
Blanket Insulation (often called 'batt') is the easiest to handle and being premanufactured, has a consistent quality. It is most suitable for application to vertical cavities (as in walls). There are two common kinds, glass fibre and mineral fibre, both with an R-value of about R-3.5 per inch (RSI-value 0.024 per millimetre)
Loose Fill Insulations are made from a variety of products and all work well for horizontal surfaces such as ceilings where the depth is not a problem. They can also be used in regular or irregular joist and wall cavities. It is essential that loose fill materials made of wood or paper products be treated for fire resistance. R-values range from R 2.5 to 3.5 per inch (RSI-values 0.016 to 0.024 per millimetre of thickness).
Rigid Insulations are made of a number of products such as polystyrene, fibreglass, urethane or isocyanurate. They are the most expensive types but do offer the highest R-values up to R-7.5 per inch (RSI-values to 0.051 per millimetre). Rigid insulations are a fire hazard when exposed to the interior but are considered safe when installed properly. In particular, they can be used on the interior of a home if covered by at least 1/2 inch (12 mm) of drywall or plaster which is mechanically fastened to the structure. Rigid insulations can be used on the outside of concrete, masonry or wood walls and under siding or stucco finishes. Some high density types are suitable for use under concrete floor slabs.
Spray-Foamed Insulations are mixed on the job site by the contractor/ installer. A liquid type foam is sprayed directly into wall cavities. The foam expands in place and sets in a short time span. Installation should only be handled by qualified installers. R-values range from R-3.5 to 6.0 per inch (RSI-values 0.024 to 0.042 per millimeter of thickness).
Sprayed-in-Place Insulations are loose fill products which are blown in to wall cavities. A mesh or plastic film is attached to the walls, the insulation is then mixed with an adhesive, usually water-based and then blown into the wall cavities. The three most common types of insulation installed in this way are cellulose, glass fibre blowing wool and mineral or rockwool. R-values range from R-3 to R 3.5 per inch (RSI-values 0.024 to 0.032 per millimetre of thickness).
The proper choice of insulation type depends on its use. In addition to high thermal resistance, a good insulation should have low absorption of water, resistance to fire, bacteria and vermin, reasonable cost, and be easily applied.
The air/vapour barrier plays the most important role in controlling air leakage heat losses and, in conjunction with caulking and weatherstripping, creates the seal between inside and outside. Exterior air barriers (taped) are recommended under any exterior siding or finish materials which are subject to air penetration
Caulking is used to seal any gaps where two surfaces meet but have limited or no movement. Most types of caulking will 'skin over' so they can be painted or are not sticky to touch when hardened.
- Oil or resin based caulks are inexpensive, but are not very durable (less than 5 years).
- Latex based materials are reasonably priced and durable, as well as being applicable to a number of different situations.
- Butyl rubber compounds are expensive but work the best for sealing wood to concrete surfaces (should only be applied in well ventilated areas).
- Elastomeric caulks (silicone and polysulphide) are very expensive but also very durable.
- Acoustical sealant, does not harden or form a skin and is used for sealing joins in the air/vapour barrier.
- Polyurethane foam is a special type of material useful for sealing large gaps around rough openings or along sill plates.
Weatherstripping is used to control air leakage at joints where two surfaces meet and move such as opening windows and doors. Weatherstripping is available in compression types, wedging types and magnet types. Good quality windows and door units are supplied with quality weatherstripping materials and are tested for air leakage rates. One should select units which have been tested and shown to have air leakage rates of less than 1/2 cfm per foot of sash length (0.80 litres per second per metre).
Polyethylene sheets are used for the air/vapour barrier. It is essential, in an energy efficient house, that the air/vapour barrier be continuous and all joints between sheets be sealed over solid backing. A non-skinning caulking such as acoustical sealant is used to seal between joints in the polyethylene. Because polyethylene is often handled roughly when being installed, 6 mil thick (0.150mm) sheets should be used. In addition to being more fragile, thinner polyethylene is much more permeable to air/vapour transmission than the thicker 0.150mm (6 mil) sheets.
The air/vapour barrier has another role to play in house construction. In addition to controlling air leakage, it prevents water vapour movement into the walls, ceilings or floors.
If vapour from the interior is allowed to enter an insulated assembly during cold weather, it could condense and form ice at some point in the wall. When the ice melts, deterioration of the insulation and structural components will occur over time. There is also a potential for supporting mold growth within the wall assembly which can cause indoor air quality problems. For this reason, the air/vapour layer must be located near the warm (or interior) side of ceilings, walls and floors.
Research has shown that as long as the air/vapour barrier is placed within the first one-third of the total assembly R-value (measured from the warm side), then no condensation problems will occur.